I "run out of room" on the spiral if I don't get the outside hand palm-to-pulse. I can't get a solid connection during the spiral.

Then, in the finishing position, if I've grabbed the outside of the wrist, my wrist is bent, my structure is past/in front of uke's arm and only my thumb is holding uke down/level. The end result is an opening where uke can resist effectively.

I also notice that students that do it with the bent wrist tend to lever on uke's elbow. Because they have more mojo under uke's wrist vice on top of it, they naturally want to lift the wrist and push down on the elbow. Owwww.